The operation of a brushless DC motor relies on a control device to input multiple PWM driving signals through power components. During operation, the brushless DC motors may experience over-current or short-circuit due to load or failure. In a case that over-current or short-circuit faults occur, large currents may be generated. The large currents may damage a main control device and even burn the power components down. If the output of the multiple PWM driving signals of the power components can be cut off timely in a case that over-current or short-circuit occurs in the brushless DC motor, it is possible to prevent a large current from causing an irreversible damage to a control device.